Use of carbon dioxide in a crude oil pipeline

ABSTRACT

A low-viscosity, low-melting point fluid liquefiable at the pressure and temperature in a pipeline and containing at least 50 percent by volume of carbon dioxide and less than 10 percent by volume of ethane is mixed with crude oil to reduce the viscosity and pour point of the oil while the oil is flowed through the pipeline. The carbon dioxide may later be separated from the crude oil and used in an oil recovery process. Natural gas produced with the oil may be used to form part of the carbon dioxide.

United States Patent [72] Inventors Ralph E. Styringi Jr.;

Leonidas P. Wharton, both of Dallas, Tex. [2 l Appl. No. 768,923 [22] Filed Oct. 18, I968 [45] Patented Aug. 3, I971 [73] Assignee Atlantic Richfield Company New York, N.Y.

[54] USE OF CARBON DIOXIDE IN A CRUDE OIL PIPELINE 16 Claims, No Drawings [52] US. Cl 55/51, 55/56 [51] lriLCl 801d 19/00 [50] Field of Search 55/44, 51,

[56] References Cited UNITED STATES PATENTS 3,389.714 6/1968 Hughes etal. l37/l3 Primary ExaminerReuben Friedman Assistant Examiner-R. W. Burks Allomeys- Blucher S. Tharp and M. David Folzenlogen ABSTRACT: A low-viscosity, low-melting point fluid liquefiable at the pressure and temperature in a pipeline and containing at least 50 percent by volume of carbon dioxide and less than 10 percent by volume of ethane is mixed with crude oil to reduce the viscosity and pour point of the oil while the oil is flowed through the pipeline. The carbon dioxide may later be separated from the crude oil and used in an oil recovery process. Natural gas produced with the oil may be used to form part of the carbon dioxide.

USE OF CARBON DIOXIDE IN A CRUDE OIL PIPELINE BACKGROUND OF THE INVENTION This invention pertains to crude oil pipeline operations, and more specifically, to the addition of a low-viscosity, low-melting point liquefied fluid to crude oil in cold areas and transporting the mixture through a pipeline. A major portion of the fluid is made up of carbon dioxide and the fluid contains less than percent by volume of ethane.

Oil, having a wide variation in properties, is produced from many different types of bituminous deposits or subsurface formations which may be remotely located in cold areas like Alaska.

In order to enhance the production of oil, it is frequently desirable to inject a fluid exhibiting some solubility in the oil, for example,,carbon dioxide. In some areas, however, there is no economical source of fluid for injection and it would ordinarily be too costly to attempt transport. of such fluids to these oil producing areas.

When oil is produced, from subsurface formations, the effluent contains oil and gas. The gasis separated from the oil. The separated gas is usually a form of natural gas containing more than fifty percent by volume of hydrocarbons, chief of which is methane. The oil remaining after separation of the gas is commonly referred to as killed crude or stock tank crude. The volume of gas in standard cubic feet per barrel of stock tank crude is commonly referred to as the gas'oil ratio which ratio varies with production history and procedure ineluding reservoir pressure and temperature. Under standard conservation practices, if the gas is richin hydrocarbons, the gas must be conserved anddisposedtof in some useful manner. ln remote producing areas, there maybenoeconomic use for the naturalgas; consequently, there arisesaproblentof how to dispose of the gasin aneconomic fashion.

Normally, thekilled crude oil is transportedin a crude oil pipeline to a refinery or to means for transportingthe crude oil to a refinery. In cold areas, the pour point and viscosityof the crude oil may be too high ,topermitthe .crude oilto be transported through a pipeline during .the cold periods of the year. When the producingarea is, remote from the refinery and the cold periods are of longduration, it may be impractical to produce crude oil fortransportation-through ,a long pipeline;

It has been proposed to transport mixtures of crude oil and lighter hydrocarbons in a pipeline. For examples, reference may be had to U.S. Pat. No. l,762,423 relating to mixing natural gas with crude oil and US. Pat; No. 3,344,583 relating to mixing a hydrocarbon containing. at least 10 percent by volume of ethane withcrude oil. This invention uses a different fluid and provides a new. use, of carbon dioxide'wherein a fluid rich in carbon dioxide is mixed with crude oil thereby solving one or all of thedifftcultiesjust described;

SUMMARY OFTHE INVENTION A low-viscosity, low-melting point'fluid containing at least 50 percent by volumeof carbon dioxide-and less than 10 percent by volume of ethaneismixed with crude-oil and themix ture is transported in a.-pipeline from onepointto another point. At pipelineconditions, the, fluid rich in carbon dioxide is a liquid and sufficiently soluble in the crude'oil to accomplish a reductionin viscosity. andnpourpoint of the crude oil. This method is especially advantageous forcrude oil pipelines where cold weather renders the-crude practically unflowablei The method also provides .a way of conserving natural gas produced with the crude oil and a source ofcarbon dioxide for use in a producing area requiringcarbon dioxidcfor injection method, the minimum amount of fluid is at least 80 standard cubic feet per barrel of crude oil. Ina third, the amount of fluid added reduces the average pour point of the crude oil to a temperature at least 5 F. below the operating ambient temperature of the coldest portion of the pipeline and maintain this pour point during brief shutdown periods.

The fluid is rich in carbon dioxide, that is, it contains at least 50 percent by volume or more of carbon dioxide. Preferably, a fluid will contain more than 75 percent by volume of carbon dioxide. If desired, the carbon dioxide may be separated at a second point and injected into a subsurface formation in a oil recovery process. In addition to being useful in an oil recovery process, carbon dioxide is especially advantageous because of its ability to mix with crude oil at cold temperatures and reduce both the viscosity and pour point of the oil and because it is a normally gaseous nonflammable fluid that has a boiling point especially suited to cold pipeline'temperatures. It has the further advantage of either existing naturally in carbon dioxide rich reservoirs, or of being present :in amounts of less than 50 percent by volume in natural hydrocarbon gases either produced separately or with the crude oil, or in being readily formed by reacting natural hydrocarbon gases or liquid hydrocarbons with oxygen of air.

DESCRIPTION OF PREFERRED EMBODIMENT This invention comprises adding at first point to crude oil a fluid containing at least 50 percent by volume of carbon dioxide and containing less than 10 percent by volume of ethane. Carbon dioxide is normally gaseous at one atmosphere of pressure at a temperature of F. and is liquefiable and luffciently soluble in crude oil at relatively low temperatures and pressures to accomplish the objectives of this invention. Preferably, the fluid will contain at least percent by volume of carbon dioxide.

The minimum amount of carbon dioxide rich fluid added to thecrude oil may be stated in terms of standard cubic feet per stock tank barrel of crude oil, or in terms of the amount of fluid required to reduce the viscosity of the crude oil by 30 percent or more, or in terms of the amount of fluid required to reduce the pourpoint of the crude'oil to a temperature of 5 F. below the lowest pipeline temperature. Generally, the amount of fluid added will reduce the pour point to at least -5 F. When the amountof fluid is givenin terms of standard cubic feet, standard conditions of pressure and temperature are used sure.- In other words, at 70 F. and atmospheric pressure the' viscosity in centipoises of the crude oil is at leastlO times the viscosity of the fluid. Since the fluid and carbon dioxide are normally gaseous at 70 F., an even greater difference in viscosity between the crude oil and fluid will exist. For example, the viscosity of carbon dioxide at 70 F. and'one atmosphere is about 0.015 centipoise which is less than onehundredth the viscosity of most crude oils.

The fluid additive is also sufficiently mixable or soluble with the crude oil to accomplish'the'aims for which the fluid is used. When the fluid richin carbon dioxide isadded to the crude oil to reduce the viscosity, or viscosity and pour point, or the crude oil, the fluid must possess the property of being sufficiently mixable with the crude oil to reduce the viscosity of the crude oil, that is, the mixture of crude oil and fluid will exhibit a lower viscosity than thecrude oil by itself. The amount of fluid added should be 'sufflcient to reduce the viscosity of the crude oil at 70 F. by 30 percent or more. For

example, if the viscosity of the crude oil is 50 centipoises at 70 F., the viscosity of the mixture of crude oil and fluid at the same temperature will be 35 centipoises or less. The degree of reduction in viscosity caused by mixing different amounts of a given fluid with the crude oil will vary with the type of crude oil. Generally, the higher the viscosity of the crude oil, the greater will be the effect of a predetermined amount of fluid provided that the oil and fluid properly mix. In all cases, however, where the fluid is added to reduce viscosity or pour point, the amount of fluid added must reduce the viscosity of the crude oil at 70 F. by at-least 30 percent. The viscosity at 70 F. of all killed crude oils tested showed a reduction of 30 percent and greater when mixed with 80 standard cubic feet of normally gaseous fluid at a pressure sufficient to fonn a solution. A procedure for measuring viscosity of mixtures of crude oil and normally gaseous fluids is given by two articles entitled Physical Properties 'of Carbonated oils," .1. R. Welker and D. D. Dunlop, Volume 228, Aug. 1963, JOURNAL OF PETROLEUM TECHNOLOGY, pp. 873876, and Generalized Correlations for predicting Solubility, Swelling and Viscosity Behavior of CO,- Crude Oil Systems," R. Simon and D. .l. Grasse, Volume 234, Jan. l965, JOURNAL OF PETROLEUM TECHNOLOGY, pp. l02- 106. The procedure of Welker and Dunlop maybe readily adapted to colder temperatures.

When the mixture of crude oil and fluid is flowed through a pipeline in a cold environment, for example, operating temperatures below 20 F. and as low as 5 F. and lower, the fluid additive must also have a low melting point or pour point. The melting point should be below 20 F. The melting point for pure carbon dioxide is about -70 F. at 5.2 atmospheres. For reasons hereinafter made more apparent, the fluid exhibits a low enough vapor pressure to be essentially a liquid at cold temperatures and reasonable pipeline pressures, that is, pressures below 500 p.s.i. For example, at 20 F., carbon dioxide is a liquid at 450 p.s.i. At cold conditions, the fluid added to the crude oil should also be sufficiently mixable or soluble with the crude oil and of such a nature as to reduce the pour point of the crude oil. The minimum amount of fluid for cold areas should reduce the pour point of the crude oil to below the ambient or operating temperature around the pipeline at the depth of burial of the pipeline. Generally, the amount added will reduce the pour point of the crude oil to at least 5 below the minimum operating temperature of the pipeline and even greater reductions are desired. Normally, the pour point of crude oil is measured at room pressure in accordance with the procedures set forth in the 1967 revision of Method 1397-66 of the American Standards for Testing Materials. Pour point measurement are affected by the thermal stress history of the oil. For this reason, three types of pour points have been measured. These are called the upper point, the lower pour point and the stabilized pour point. Of these, the upper and lower pour points are still included in Method D97-66. Both are significant in the handling of crude oil in cold areas. There are no standardized methods for measuring pour points at elevated pressures. For example, the types of pressures that would be required to hold normally gaseous carbon dioxide in solution with crude oil. The pour points or crude oil-normally gaseous carbon dioxide rich fluid mixtures of this invention were measured at elevated pressures using a 150 milliliter sample bomb containing four, Vs-inch, nickel balls. Measurements were made using 100 milliliters of crude oil mixed with a preset amount of fluid additive. The bomb was cooled in dry ice at a rate of about 3 F. per minute. Every few minutes the bomb was shaken to determine if the balls could be moved. The temperature at which the balls froze in place is used as the pour point. An average of two or more measurements is used. The'number of measurements made depends upon the variation between tests. Procedures for high and low pour point similar to those set forth in ASTM D07-66 were conducted using this bomb. The amount of reduction in pour point caused by mixing a given amount of fluid with crude oil is dependent upon the properties of the fluid and the crude oil. For

a normally gaseous fluid like carbon dioxide, it was found that at least standard cubic feet of fluid perstock tank barrel of crude oil must be added to substantially reduce the pour point of the crude oil. For the colder temperatures higher amounts will be needed. For example, in samples of crude oil having an average pour point of about 0 F., the pour point was a =1 5 F. when a mixture corresponding to 230 s.c.f. of carbon dioxide per barrel of oil was tested. Carbon dioxide is soluble enough in crude oil at low pipeline temperatures to permit these amounts of carbon dioxide to be mixed with crude oil at reasonable pipeline operating pressures.

When the fluid containing normally gaseous carbon dioxide is added to crude oil to reduce the pour point of the crude oil,

it is especially desirable to maintain the pressure of the mix-' ture above the bubble point of the solution during those periods when flow the mixture between pumping or booster stations is interrupted. During these period, if the pressure were pennitted to decrease below the bubble point of the mixture, some of the normally gaseous carbon dioxide or other constituents of the fluid would vaporize from the solution. This would decrease the concentration of dissolved fluid in the crude oil and this, in turn, would increase the pour point of the mixture. if the pour point increased to a temperature above the operating temperature of the pipeline, the crude oil. would congeal causing a great deal of difficulty. The necessary pressure. is usually maintained by pressure applying means and valves; however, there may be instances where these pressure maintenance systems will not function. in such situations, there should be more than enough normally gaseous fluid in the mixture to lower the pour point to the desired level. With an excess of normally gaseous fluid present, partial vaporization of the fluid will maintain the pressure in the pipeline at the bubble point of the solution thereby holding enough carbon dioxide in the crude oil to maintain the pour point to a level below the operating temperature of the pipeline. As a result, the crude oil would not congeal during the shutdown period. When the movement of the mixture in the pipeline is resumed, the oil will still be flowable and the vaporized fluid will redissolve or reliquify. For similar reasons, an excess of normally gaseous fluid should be added when the fluid is sued to reduce the viscosity of cold crude oil to render it pumpable.

One advantage in the use of carbon dioxide as a major portion of the fluid is that this material of carbon dioxide may be at least partially generated from natural gas. The natural gas may either be produced separately from gas reservoirs or the natural gas may be obtained by separating the gas from the produced crude oil. The fluid rich in carbon dioxide can be produced by separating natural carbon dioxide from at least a portion of the hydrocarbons in the natural gas or by burning the natural gas to form a combustion gas rich in carbon dioxide either with air or oxygen. If air is used in the burning process, the carbon dioxide may be separated from the combustion gas by conventional processes, such as, the monoethanolamine and hot carbonate process, or some other known separation process. The carbon dioxide rich fluid may then be compressed, liquefied and blended with the crude oil in the pipelines itself. Even if the volume of carbon dioxide is in excess of that soluble in the oil, at cold conditions it is practical to operate with liquid carbon dioxide dispersed in the crude oil.

As mentioned previously, the mixture of fluid and crude oil is flowed through a pipeline to a second point. This second point may be adjacent or near a producing area requiring a source of carbon dioxide for use as an injection fluid in an oil recovery process. in this situation, the use of carbon dioxide as the additive fluid has a still further advantage. The carbon dioxide may be separated from the crude oil-fluid mixture in the pipeline using methods similar to those set out in US. Pat. No. 3,344,583. At least part of the separated carbon dioxide may then be injected into a subsurface formation in any oil recovery process wherein a gaseous fluid, especially carbon dioxide, would be useful.

There have been shown, described and pointed out the fundamental novel features of this invention. It will be understood that various omissions, substitutions and changes in the form and details of the described processes may be made or supplied by those skilled in the art of crude oil production and transportation without departing from the spirit of the invention. It is, therefore, our intention to be limited only as required by the scope of the appended claims and reasonable aspects thereof.

We claim:

1. A method of transporting crude oil in a pipeline comprising adding to crude oil at a first point a fluid containing at least 50 percent by volume of carbon dioxide and less than ten percent by volume of ethane to form a mixture of said crude oil and said fluid, said fluid being a liquid at the temperature and pressure in said pipeline, said fluid being further characterized in that said fluid has a viscosity at a temperature of 70 F. substantially less than said crude oil at said temperature and will mix with said crude oil to reduce the viscosity of said crude oil and the amount of said fluid being sufficient to form a mixture having a viscosity at said temperature at least 30 percent less than the viscosity of said crude oil at said temperature, and flowing said mixture through said pipeline to a second point.

2. The method of claim 1 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.

3. The method of claim 1 wherein at least a part of the carbon dioxide portion of said fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.

4. The method of claim 3 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.

5. The method of claim 1 wherein the fluid added to the crude oil contains at least 75 percent by volume of carbon dioxide.

6. The method of claim 5 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.

7. The method of claim 5 wherein at least a part of the car bon dioxide portion of said fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.

8. The method of claim 7 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.

9. A method of transporting crude oil in a pipeline comprising adding to crude oil at a first poin't fluid containing at least 50 percent by volume of carbon dioxide and less than 10 percent by volume of ethane to form a :mixture of said crude oil and said fluid, said fluid being a liquid at the temperature and pressure in said pipeline, said fluid being further characterized in that said fluid has a viscosity at a temperature of 70 F. substantially less than said crude oil at said temperature and will mix with said crude oil to reduce the viscosity of said crude oil and the amount of said fluid being at least standard cubic feet per barrel of said crude oil in said mixture; and flowing said mixture through said pipeline to a second point.

10. The method of claim 9 wherein natural gas is used to form at least part of the carbon dioxide portion of aid fluid added to said crude oil.

11. The method of claim 9 wherein at least a part of the car bon dioxide portion of said fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.

12. The method of claim 11 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.

13. The method of claim 9 wherein the fluid added to the crude oil contains at least 75 percent by volume of carbon dioxide.

M. The method of claim 13 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.

15. The method of claim 13 wherein at least part of the carbon dioxide portion of sad fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.

16. The method of claim 15 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil. 

2. The method of claim 1 wherEin natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.
 3. The method of claim 1 wherein at least a part of the carbon dioxide portion of said fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.
 4. The method of claim 3 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.
 5. The method of claim 1 wherein the fluid added to the crude oil contains at least 75 percent by volume of carbon dioxide.
 6. The method of claim 5 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.
 7. The method of claim 5 wherein at least a part of the carbon dioxide portion of said fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.
 8. The method of claim 7 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.
 9. A method of transporting crude oil in a pipeline comprising adding to crude oil at a first point fluid containing at least 50 percent by volume of carbon dioxide and less than 10 percent by volume of ethane to form a mixture of said crude oil and said fluid, said fluid being a liquid at the temperature and pressure in said pipeline, said fluid being further characterized in that said fluid has a viscosity at a temperature of 70* F. substantially less than said crude oil at said temperature and will mix with said crude oil to reduce the viscosity of said crude oil and the amount of said fluid being at least 80 standard cubic feet per barrel of said crude oil in said mixture; and flowing said mixture through said pipeline to a second point.
 10. The method of claim 9 wherein natural gas is used to form at least part of the carbon dioxide portion of aid fluid added to said crude oil.
 11. The method of claim 9 wherein at least a part of the carbon dioxide portion of said fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.
 12. The method of claim 11 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.
 13. The method of claim 9 wherein the fluid added to the crude oil contains at least 75 percent by volume of carbon dioxide.
 14. The method of claim 13 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil.
 15. The method of claim 13 wherein at least part of the carbon dioxide portion of sad fluid is separated from the mixture at the second point and is injected into a subsurface formation in an oil recovery process.
 16. The method of claim 15 wherein natural gas is used to form at least part of the carbon dioxide portion of said fluid added to said crude oil. 